In the processing of semiconductor substrates (e.g., wafers), plasma is often employed. In plasma processing, the wafers are processed using a plasma processing system, which typically includes a plurality of processing modules. The substrate (e.g., wafer) is disposed on a chuck inside a processing module during plasma processing.
In order to move a wafer in and out of the process module, the wafer is typically placed on an end effector and transferred onto the chuck. The end effector is a structural component configured for supporting the wafer during wafer transfer. The end effector is typically disposed on a robot arm. FIG. 1 shows a representative prior art end effector 102 for supporting a wafer 104 during wafer transfer. For illustration purposes, a portion of a robot arm 106 is also shown.
Generally speaking, during a wafer transfer sequence, the robot arm first moves the end effector to pick up the wafer from a wafer storage cassette or station. Once the wafer is positioned on the end effector, the robot arm would then move the wafer into the plasma process module through a door in the processing module. The robot arm then positions the end effector and the wafer over the chuck and then places the wafer on the chuck for plasma processing.
In order to ensure that the wafer is processed properly (thereby ensuring controllable and repeatable process results), the wafer needs to be centered on the chuck during plasma processing. If the end effector is perfectly centered relative to the chuck and the wafer is perfectly centered relative to the end effector, then the wafer would be perfectly centered relative the chuck when the robot arm places the wafer on the chuck.
From the robot controller's perspective, it is important to know the center of the chuck to enable the robot controller to center the end effector over the chuck for the purpose of wafer placement. Accordingly, for any given plasma processing module, the robot controller needs to be taught the position of the chuck and the chuck center. In other words, the robot controller needs to ascertain in its own coordinate system the exact location of the chuck and the chuck center since each chuck may be positioned slightly differently in each processing module due to, for example, machining and/or manufacturing and/or assembly tolerances.
To compensate for the end effector/chuck mis-alignment, the typical strategy during calibration involves moving the robot arm to a position where the center defined by the end effector (herein referred to as the “end effector center” or the “end effector-defined center”) actually aligns with the center of the chuck. To accomplish end effector calibration, it is necessary that the operator be able to ascertain the actual end effector/chuck alignment position. In the prior art, the alignment of the end effector center to the chuck center is accomplished using a fabricated mechanical fixture which fits on the edge of the chuck or attaches to the process module interior. The mechanical fixture has a key feature (essentially a centering protrusion for the end effector), which allows the end effector to rest right up against the key feature of the calibration fixture. Since the fixture is centered relative to the chuck, when the end effector rests against the key feature of the fixture, the end effector center would be centered on the chuck. Typically, positioning the end effector against the key feature is accomplished by an operator pulling or pushing the end effector against the key feature so that the end effector rests against the key feature.
After the operator has positioned the end effector against the key feature, the operator then registers the robot arm position with the robot control system so that the robot control system can record, in the robot control's coordinate system, the position of the robot arm that achieves this actual end effector/chuck alignment.
During production, the robot arm moves the end effector to the coordinates associated with this effector/chuck alignment position. If the wafer is centered with respect to the end effector, the fact that the end effector center now actually aligns with the chuck center would cause the wafer to be centered relative to the chuck when the wafer is placed by the robot arm on the chuck for wafer processing.
However, there are disadvantages with the prior art technique for centering the end effector relative to the chuck for calibration purposes. First of all, there are many types of chucks and processing modules in existence. Therefore, in order to use the mechanical fixture approach to perform calibration, many different mechanical fixtures must be fabricated and stocked. Also, affixing a physical mechanical fixture, which may have one or more hard metal edges or surfaces, on the chuck may potentially damage the chuck. Additionally, if this calibration is done in the field after some plasma cycles have been executed in the processing module (e.g., in response to a concern that the end effector may not be centered relative to the chuck following a production run), the attachment of a physical calibration fixture on the chuck may cause deposited particles on or near the chuck to flake off into the processing chamber. During the subsequent processing cycles, such particles constitute particle contamination, which is undesirable.
Additionally, because the calibration is performed at atmospheric pressure, the prior art calibration technique may not effectively duplicate the conditions that exist during production. This is because during production, components of the processing module may be placed under vacuum, causing one or more components to shift due to the pressure differential between the vacuum environment and the ambient atmosphere. Since the calibration conditions do not faithfully duplicate the production conditions, accurate calibration may not be possible.
Furthermore, if the positioning of the end effector at the end effector/chuck alignment position is performed manually (e.g., involving the operator pulling or pushing the end effector to rest up against the key feature of the mechanical fixture), there may be a shift in the robot arm position when the operator releases the robot arm to go and register this end effector/chuck alignment position with the robot controller. This shift may occur for many reasons, including for example the fact that the robot motors are de-energized. When the robot arm pulls away, even by a small amount that may be imperceptible to the robot operator, this shift may result in inaccuracy in the calibration process. If the calibration process is inaccurate, inaccurate wafer placement during production may occur, leading to decreased yield and an increase in the rejection and/or failure rate for the fabricated products.